An inherent advantage of a permanent magnet machine is the good efficiency because permanent magnets generate magnetic field without losses in contrast to electromagnets. On the other hand, a permanent magnet machine is not free from challenges when the permanent magnet machine is a marine shaft generator or an electrical machine of another system where the electrical machine cannot be separated from a mechanical power line even in a case of a winding fault. In conjunction with typical permanent magnet machines, one of the challenges is related to the fact that the magnetic field generated by the permanent magnets induces voltages to the stator windings also during a fault, e.g. a turn-to-turn fault, of the stator windings when the rotor of the permanent magnet machine is rotating. This may cause fault currents which, in turn, may lead to a risk situation.
A straightforward approach would be to keep the permanent magnet machine stopped, i.e. non-rotating, after the fault has occurred but in all cases this is not possible. For example a propulsion shaft of a ship is allowed be stopped only for a relatively short time defined by regulations. Thus, a permanent magnet shaft generator should be deactivated within the relatively short time during which the propulsion shaft is allowed be stopped. There are basically the following two options to deactivate the permanent magnet shaft generator: a) disconnecting the rotor mechanically from the propulsion shaft and b) eliminating the magnetic field caused by the permanent magnets. Disconnecting the rotor mechanically from the propulsion shaft is a demanding task and might take a too long time depending on the construction and space available. Normally, eliminating the magnetic field caused by the permanent magnets, i.e. demagnetizing the permanent magnets, requires heat and/or strong external counter acting magnetic field or combination of these two. This, of course, would also destroy the permanent magnets.